"Elemental Affinity: After each rest, roll a d10 to determine your current elemental affinity. You can choose the damage type when you use your second wind, but the type changes again when you rest. On your turn, when you use an arcane attack power that deals force, necrotic, poison, or psychic damage, you can change the damage type to match that of your current elemental affinity. Your elemental affinity’s damage type replaces each instance of force, necrotic, poison, and psychic damage that the power deals."

Provided Wizard / Warlock hybrid takes the pact - Does this change even the wizards spells, such as Magic Missile etc?Can Accursed Affinity be spread around with the curse? (Not sure what it is that spreads the curse around)

"On your turn, when you..." means that the spells out of my turn are the original damage types?

And for the wizard part: If I move stinking cloud on an enemy in my turn, do they get damaged when sliding over them, or just when they actually start their turn in? If so, how many times can I apply the damage? And what if the NPC is dragged into the cloud by slide power on my turn?

"Elemental Affinity: After each rest, roll a d10 to determine your current elemental affinity. You can choose the damage type when you use your second wind, but the type changes again when you rest. On your turn, when you use an arcane attack power that deals force, necrotic, poison, or psychic damage, you can change the damage type to match that of your current elemental affinity. Your elemental affinity’s damage type replaces each instance of force, necrotic, poison, and psychic damage that the power deals."

Provided Wizard / Warlock hybrid takes the pact - Does this change even the wizards spells, such as Magic Missile etc?

It appears that, yes, you could also choose to change the damage type on your wizard powers as well, because nothing in hybrid warlock or the pact description says it applies only to warlock powers.

Can Accursed Affinity be spread around with the curse? (Not sure what it is that spreads the curse around)

What do you mean by spreading Accursed Affiinity? You use your Warlock's Curse at-will power to curse enemies as a minor action. You can curse multiple enemies at a time, but only do the extra damage from your curse once per turn. The tricky part is that you can't apply your curse to a creature that is already cursed, and Accursed Affinity applies the benefit only to the first creature you curse after killing another creature you had cursed; so it would be to your advantage to not curse multiple enemies. Note that if you go the hybrid route, you need to take Hybrid talent feat to gain Accursed Affinity, and you can only gain the bonus damage from your curse with Warlock powers, but the Vulnerability from your accursed affinity applies to anyone's attacks.

"On your turn, when you..." means that the spells out of my turn are the original damage types?

Yes, so you can not use the pact feature with any immediate interrupt (including readied actions), immediate reaction, oppurtunity action, or free actions granted outside your turn.

And for the wizard part: If I move stinking cloud on an enemy in my turn, do they get damaged when sliding over them, or just when they actually start their turn in? If so, how many times can I apply the damage? And what if the NPC is dragged into the cloud by slide power on my turn?

Not 100% sure on this, but I believe that if you move the cloud, they only take damage at the start of their turn, because they did not "enter" the cloud. If they are moved by forced movement into the cloud, then they take the damage, but only once per turn (so you can't slide them in and out repeatedly)